First and most important, no one cares what happens at any level beyond the maximum level after a month, which is roughly the time when subscriptions kick in. This means it is essential that you elder game is near perfect on release while bugs or less interesting periods during leveling are tolerable. As long as you are leveling, you keep chasing the max-lvl number, bugs can be annoying but when not gamebreaking will be bypassed by players. On the other side when players reach max level and the elder game is absent, infuriatingly broken or does not dangle the same carrot on a stick before them as leveling does, players will quit, having beat the game or simply being frustrated. Your ideal target audience, the stable subscriber spends days to weeks leveling and years at max level never again venturing out to the low level regions of the world, which in other MMO's was most of the world. This is where both AoC and more recently SW:TOR went awfully wrong. The "Tortage-Syndrom" of AoC in which the first 20 levels are fully voiced, involving and fun while the rest of the game is text based, bland and obviously unfinished (at the time of release) was probably the the most severe case. SW:TOR focused hugely on story which people like me blasted through in 5 days and people with a life took about a month to finish. What followed was unfinished unbalanced PvP that felt like it could have been good but was messed up and Raids in which all three difficulty settings where almost equivalent and were again cleared in a few additional weeks. So the "casual" players leave after roughly 1-2 months, after reaching max level and going through raids at lowest difficulty, while the "hardcoe" players too leave after about that time because they reached max level earlier, cleared nightmare and beat the game (a game that did not pose a particular challenge or reward for choosing a more difficult setting.). PvP on the other hand remained broken and exploitable for too long, the high level rewards were obtained from a warzone in which both fractions needed to destroy objectives. Instead of sparking an all out war, players from opposing faction cooperated, taking turns in destroying their objects and farming items by a klick rather than playing a game.(Interesting from a human behavior point of view is that this cooperation required no communication. Getting items is more important than doing PvP and everybody agreed on that a priori). So if you want box sales, hype the game and build a great leveling experience, but if you want a stable subscription base and to avoid having to convert to F2P later, the Hamster wheel needs to be in place at launch. There must be a difficulty ceiling in PvE that players can't or at least or not expected to pass, there must be a finished gear progression for endgame that will take weeks or a few months depending on the player and both PvE and PvP must be thoroughly tested and balanced. There is no way you can come up with Raids and PVP before release and, as in SW:TOR's case, go "yes we know the importance of the elder game", put out the crap we got from them and then be surprised when subs plummet after 3 months. The hamster wheel of endgame PvE and PvP is what keeps your subs and shapes your community.
Secondly, understand how MMO communities work. In general, internet communities are very unpleasant things, some more than others. I'd like to mention that I have mainly been playing LoL recently. I can think of two mindsets that drive community formation in a MMO, need based and socializing. A need based community arises because I can't kill that dragon alone. If I could I would, I'd take all the loot and would not have to deal with unpunctual players, players that mess up (even though I mess up just as much, I wouldn't add additional potential for failure if I could avoid it) or the general E-drama that guilds and raiding groups bring with them. However because I need other people to accomplish my personal goal, kill the dragon, I put up with this and try to find the least frustrating bunch of allies that I have common goals with. As in the above example, cooperation of players from opposing factions instead of conflict, I don't enter a dungeon with 24 other players because I like to PvE with my friends, I do it because I want item X from boss Y and I need the help of other players for that. These communities break apart when there is no common goal or no common enemy. If there is no content to progress through for a few months, there is no reason for me to put up with the hassle of being in a raid. Furthermore these communities exist because the of selfishness of individuals. Everybody wants to kill a certain boss or clear raids for their items, their bragging rights, their [censored]. To be satisfied, these people (including me) need to walk through social hubs as the only one dressed in the best armor ingame or riding whatever rare mount drops from the last boss. WoW is a great example here for both good and bad design choices. Introduction of high and low difficulty settings meant that most all players could see the current raid instead of only the few percent that made it to e.g. Kel'thuzad in classic. Therefore development time was not wasted on only a few "hardcoe" players and "casuals" got a longer game experience, i.e. they wouldn't quit after finding out that they can't progress further without investing much more time. However this immediately lead to a loss of incentive for "hardcoe" players. The items found in easy mode were graphically almost identical and while moderately inferior to their hardmode counterparts performance-wise, still very viable. Because high level items of comparable performance and graphic style were now available to everybody this has a severe impact on the bragging rights of "hardcoe players". (Think: "I just spent a week finding a way to kill boss X and the 10 guys over there did it on easy mode in two hours. Still I look like them. Why? Whats the point of gear then?). So what you need is regular content updates and multiple difficulties that are rewarding in their own right. Give ok-ish rather bland looking items for easy mode, give exceptional and very shiny items(not green to red reskins of easy mode items) for hard mode encounters. "Casuals" will have content to progress through, "hardcoes" will have something worthwhile to work towards.
The second mindset is socializing. What WoW and MMOs in general recently seem to have forgotten in their quest for more subscriptions is that while the socializing mindset generates a much more pleasant community than a need driven community, it requires a need driven community to form. Socializing people are drawn to existing communities and they flee from shrinking communities. I feel that designers are under the delusion that they cater content to socializing people by adding gestures and emotes and collectibles and non-combat events. I have never seen anybody stroll casually through the Darkmoon Faire or go fishing for fun in WoW. One caters to socializing communities by providing a stable, need based community on which to grow. If you loose the stable need based community, you loose the socializing community, the people that have actually joined to chat or play with their friends.
Thirdly, crafting. In all MMO's I have played crafting is utterly useless, most recently SW:TOR, where everybody would pick biochem beacuse it provided reusable medpacks and a constant HP boost. WoW converted its crafting system in a similar fashion to provide unique buffs based on skill level. In these cases one only pushes ones crafting skills to min-max ones char and it is not optional. Only very few professions are able to generate sell-able items while every profession includes hundreds of recipes that will never be made, design time better spent elsewhere. If I have to decide on farming materials and skilling up smiting for a hour to make a shield or getting a shield of the same quality by talking to a guy and killing 5 birds in 10 minutes I wont be crafting. Find a way to make crafting useful or don't waste your time on it. You could for example introduce quality levels for items that increase with skill. Say both a lvl1 and a lvl 300 smith can make a daedric shield but the quality and thereby stats of the shield the lvl 300 crafter makes is far superior. Thus any player could pick up a crafting skill and making items would not have a skill requirement. Crafting skill can then rise *much* slower (so slow that no one power-levels it, say over the course of weeks or months) allowing the items a crafter makes to actually reflect the amount of effort he has put into crafting.
This would allow some crafters to stand out of the crowd their. Take WoW where items can be socked with gems from jewelcrafting. Many jewelcrafters existed on my server, throwing the same items, say a +10 crit gem into the AH. I remember one guy in particular that knew all recipies (even the useless ones), crafted for at least two years, was known for and seemed to take pride in his craft which surprised me because I could level up jewel crafting and make the jewels I needed in about 2 days. In a system where skill reflects effort spent and produces high quality items, this guy may have been able to eventually make a +11 or +12 crit gem and people would actually seek him out specifically rather than just grabbing the cheapest stuff from the AH, giving his crafting meaning beyond role play.
Lastly, one of the biggest horrors in MMO gaming I have encountered are instanced or sharded worlds, i.e. parallel existing copies of an area. In every game I have played that included this, the game felt dead, artificial and it completely ruined immersion. I just wanted to shortly mention it, in interviews you stated you were going for 200 people on one screen and for non-instanced dungeons so I guess you have a similar opinion, but when SW:TOR promised to use it only sparsely and ultimately delivered a game in which most planets were spawned in shards and even after release I would only run into other players occasionally, that was a moment of "I might quit this...it doesn't feel like a MMO at all". Don't shard.
Yes, I have no research or citations to support my claim, I am not a psychologist this is just my opinion. And yes, this must sound pretty bleak for the average role-player who fancies himself a dagger wielding wood-elf. But this is supposed to be a MMO, a game that needs to get people hooked and requires basic game-play features compatible with an audience that is immature and egocentric. I include myself here. Yes you may be different and I generalize a lot. But if you go to what ever WoW's current capital is or play really anything online you know internet gaming is a place mainly for a very special group of people that do not log into a game to build houses, use their dance emotes, exchange pleasantries and marvel at the landscape the design team has created.
Anyhow, looking forward to the game

*edit* hi! yes this is a wall of text. If you would like to comment on this wall of text by saying that it is a wall of text go ahead, maybe also mention the font or the background color of the forum. Please try to come across as peeved and witty as possible because I am forcing you to read this as you have no free will and everybody else will find your comment original and hilarious.